Newark Castle was founded in the early twelfth century by Alexander, bishop of Lincoln, beside the River Trent and the Great North Road. Its position allowed the bishop to supervise a crossing, collect revenue, and maintain a secure residence between Lincoln and the Midlands. Stone construction replaced earlier earth-and-timber defenses, producing a riverside enclosure with towers, halls, chapels, service buildings, and a substantial gatehouse. Although created by a churchman, Newark functioned as a political and military center as well as an episcopal household.

The Crown took control during disputes between Alexander and King Stephen, and the castle remained entangled in royal politics. Its most famous medieval episode came in October 1216, when King John arrived while retreating during the First Barons' War. Ill and having lost part of his baggage in the Wash, John died at Newark on the night of 18–19 October. His body was taken to Worcester for burial. Later bishops recovered the property, repaired the buildings, and used its apartments during travel and administration.

By the seventeenth century Newark's location made it a key Royalist stronghold controlling routes and communications in the East Midlands. The town and castle endured three parliamentary sieges during the English Civil War. The first two, in 1643 and 1644, failed to force a permanent surrender. Defenders created earthworks around the town and used the medieval castle as one part of a much larger fortified system. Soldiers, townspeople, refugees, animals, and supplies crowded within defenses while disease and shortages accompanied the repeated campaigns.

The final siege began in late 1645. A combined English and Scottish force tightened its lines around Newark, but the garrison held until King Charles I ordered surrender in May 1646 after placing himself in Scottish hands. Parliament directed that the castle be slighted so it could not again serve as a fortress. Much of the interior was demolished, roofs disappeared, and the surviving riverside walls and gatehouse became a monumental shell. The broader siege works were gradually absorbed into fields, roads, and later urban development.

Newark's castle grounds were eventually adapted as a public garden, while archaeology and landscape study identified traces of the Civil War defenses beyond the medieval enclosure. Conservation now treats the standing ruin and scattered siege sites as parts of one military landscape. The site connects episcopal power, King John's final journey, and one of the longest Royalist defenses of the Civil War. Its surviving wall beside the Trent can appear picturesque, but the missing ranges and earthworks are equally important evidence of the dense administrative, residential, and wartime activity that once filled the area.